The performance of thermoelectric materials is mainly governed by the materials’ electrical and thermal conductivity properties and a number of new materials and structures have been exploited in order to optimize the energy conversion efficiency. Especially, nanostructure engineering via dopants, precipitates or phase/twin/grain boundaries is found to be effective in increasing the conversion efficiency by reducing the thermal conductivity. However, a direct correlation of these nanostructures to the material’s property is yet to be elucidated. Nowadays, with the rapid development of aberration-corrected transmission electron microscopy (TEM), the resolution of electron microscopes takes a leap forward to sub-angstrom and sub-eV, which allows a direct access to a material’s structure and chemical composition at an atomic scale.
In this talk, we present the atomic and nano structure of layered thermoelectric material AgCrSe2, By using the state-of-the-art aberration-corrected electron microscopy, we characterized its structure and found that it has an aperiodic stacking of AgCrSe2 unit cells and bilayer Ag atoms in the order of …-(AgCrSe2)m-Ag-Ag-(AgCrSe2)n-Ag-Ag-…, in which the interface between AgCrSe2 and bilayer Ag atoms are incoherent and the average distance between these interfaces is below one unit cell. According to the Anderson localization theory, these high-density aperiodic incoherent interfaces would lead to the localization of phonon and thus result in the extremely low lattice thermal conductivity. In addition to the stationary structure characterization, the heat-driven dynamic behavior of the thermoelectric materials is another important aspect to investigate, as the performance of the thermoelectric materials is usually temperature-dependent. In-situ Cs-corrected TEM technique is an ideal tool for directly probing the local structural change of layered thermoelectric materials with ultrahigh resolution. We have conducted in-situ heating experiment on various layered thermoelectric materials, where we are able to monitor the structural evolution as a function of temperature. By correlating the change in the microstructure and their bulk thermoelectric properties, we gain insight of the origins of their extraordinary high ZT performance.